The invention relates to a device for transmitting a plurality of television signals through a transmission channel. The invention also relates to a device for storing a television signal and a device for playback of said signal.
For future Digital Video Broadcast (DVB) systems it is being considered to transmit a plurality of television signals simultaneously through one digital transmission channel. A device used for this purpose comprises an encoder for each television signal for encoding this signal in a bit stream, control means for controlling the bitrate of each bit stream and means for combining the bit streams to a channel bit stream.
Inter alia, the MPEG2 standard which is being developed is suitable for encoding each television signal. An encoder used for this purpose is described in ISO/IECCJTC1/SC29/WG11/N0400 xe2x80x9cTest Model 5xe2x80x9d, April 1993. The known MPEG2 encoder forms a bit stream for the television signal, whose bitrate is controlled in conformity with a target value. If there is a risk that the target value is not achieved, for example when complex picture scenes occur, the picture is quantized in a coarser way in order that fewer bits per picture are generated. In practice the picture quality of the encoded television signal will consequently vary from scene to scene and may even be noticeably poor.
When transmitting a plurality of television signals through one digital transmission channel, the available channel bitrate is divided into a plurality of bitrates, one of which is allocated to each television signal. The allocated bitrates have been fixed in advance. For each of the transmitted television signals it now holds that the picture quality of complicated picture scenes may be subject to degradation.
It is an object of the invention to provide a device for transmitting a plurality of television signals through a transmission channel with which a higher picture quality is achieved.
To this end the device according to the invention is characterized in that the control means are adapted to control the bitrate of each signal in dependence upon the complexity of said signal with respect to the joint complexity of the plurality of television signals.
The invention is based on the recognition that upon transmission of a plurality of television signals through one transmission channel not all television signals will be simultaneously complicated. For example, if two television signals A and B are transmitted and if, at a given moment, signal A comprises a scene which is not so complex and signal B comprises a very complex scene, signal B will be temporarily transmitted at a higher bitrate at the expense of the bitrate of signal A. This process of dynamic allocation of bitrates will hereinafter be referred to as xe2x80x9cjoint bitrate controlxe2x80x9d.
In known stand-alone encoders, for example MPEG encoders, each encoder has its own control means for achieving a predetermined target value of the bitrate. An embodiment of a device on this basis comprises a common adjusting circuit for applying a bitrate target value to each of the control means in dependence upon the complexity of the corresponding television signal. The complexity can be measured by a preanalysis circuit, for example in the form of an additional encoder. The complexity may also be computed from the number of bits and an average step size with which a previous picture of said television signal has been encoded. This possibility has the advantage that such a computation is already possible in MPEG encoders.
A further embodiment of the device is characterized in that the control means are adapted to form a joint control value for each encoder in dependence upon the joint complexity of the plurality of television signals. In this embodiment the bitrate of each bit stream is directly representative of the complexity of the signal. Therefore, the complexity need not be measured separately.
It has been found that joint bitrate control reduces the risk of occurrence of a noticeably poor picture quality in each of the signals. The picture quality of the transmitted television signals thus roughly increases. However, joint bitrate control also provides the possibility of transmitting more television signals through the same transmission channel while maintaining the picture quality. It has been found that five television signals can be transmitted through a 20 Mbit/s channel at the same picture quality as four television signals having a constant bitrate of 5 Mbit/s.
Further attractive possibilities of using joint bitrate control are obtained by dividing a television program into a plurality of parts of equal length and by simultaneously storing these parts as one channel bit stream on an optical disc by means of joint bitrate control. The television signal is reproduced by repeatedly reading the stored channel bit stream and by each time decoding a different part. Either a longer playing time or a higher picture quality is then obtained.